The present invention relates to pivotally mounted members and specifically concerns a device for controllably positioning a reflective member utilizing electromagnetic forces. Such devices are particularly useful in optical image reproducing systems which read a video disc and develop an electrical signal representative of stored program information.
Systems for the retrieval and reproduction of information stored on rotating video discs are now fairly well known, for example, from U.S. Pat. No. 3,854,015. A video disc has information stored as concavities or pits in a spiral shaped track. The length of the concavities and the spacing between them determine the information on the video disc. An optical reading beam, for example, laser generated, impinges on the concavities and follows the track to read the information. The concavities affect the light of the reading beam so that the light reflected from the disc is modulated in accordance with the stored information.
Because there is no mechanical coupling between the disc and the reading beam, radial tracking misregistration can occur. That is, the optical beam may drift between tracks and information will not be desirably reproduced. Accordingly, arrangements are known which develop an error or tracking correction signal having a polarity which indicates the sense of radial misregistration and an amplitude proportional to the extent of misregistration, as described in U.S. Pat. No. 3,854,015.
A variety of devices have been employed to respond to the correction signal and adjust an optical member, such as a mirror or a prism in the optical path of the read beam, to maintain tracking registration. Additionally, adjustment of such optical members is utilized for deflecting the optical beam to correct timing errors introduced by fluctuations in the rotational speed and by mechanical distortions in the disc. The read beam is corrected through a tangential path normal to the beam path required for radial correction.
Prior art deflectable devices relevant to the present invention are of two types. The first type employs permanent magnets fixed to a mirror pivotally mounted on a frame and surrounded by a coil fixed to the frame. The error correction signal is applied to the coil leads and the mirror is electromagnetically deflected from its rest position in proportion to the applied signal in a manner well-known in the art. This type of arrangement, however, is subject to stray magnetic fields which can cause undesired movement of the mirror.
A second type of deflectable member device includes magnets or pole pieces fixed to a frame and a coil fixed to the underside perimeter of the mirror. The mirror is pivotally mounted to the frame by means of a support having a first end fixed to the frame and a second end cooperating with the underside surface of the mirror. An elastic or resilient element extends between the mirror underside surface and the support second end and provides adhesion of the mirror to the support and permits the pivotal motion of the mirror. The coil and the mirror are then arranged in the magnetic field produced by the pole pieces. The mirror is pivotal about the support in a direction and magnitude controlled, respectively, by the polarity and magnitude of the error correction signal applied to the coil. Further, the resilient element provides forces which restore the mirror to its rest position when the error signal has zero amplitude.
This second type of device eliminates the stray magnetic field problems but results in mechanical resonances produced by the coil lead wires of the moving coil. Attempts have been made to reduce such lead resonances, through, for example, alignment of the coil leads spaced from and parallel to the mirror's pivotal axis. Applicant believes such a prior art device is presently contained, for example, in the model No. VP1000 Video Disc Player manufactured by the Pioneer Corporation. These attempts, however, have not proven entirely satisfactory. The alignment approach is fragile and difficult to maintain. If the lead wires are slightly off center, they tend to bend as the element pivots and eventually break.